The present invention relates to a method for compensating bending of a lip beam in a paper machine, when papermaking pulp flowing through a slice formed with the lip beam, causes loading upon the bottom surface of the top lip beam. The present invention also relates to apparatus in accordance with this method.
Reference is made to Finnish Pat. No. 50,156, corresponding to U.S. Pat. No. 4,008,123, as the state of the art, this reference describing loading of the front wall beam of a headbox of a paper machine or similar device, and particularly describing supporting intended to compensate for the temperature deflection. In this reference, the front wall beam is supported at least at two points, preferably at the so-called Bessel points, on the frame beam of the headbox.
According to this Finnish patent, in order to compensate for the deflection caused by the loading of the front wall beam and the temperature difference between the inner wall and the outer wall, adjustable power equipment has been arranged onto the ends of the front-wall beam. Although it is possible with the solution described in Finnish Pat. No. 50,156 to considerably reduce the bending of the top lip beam as compared to the situation when the beam is supported only at its so-called Bessel points, it is, in practice, not possible to achieve a sufficiently even slice with this solution.
Reference is also made to Finnish patent application No. 82-4439 as the state of the art, which relates to an arrangement in the supporting system of the top lip design in the headbox of a paper machine. In this arrangement, the shape variations of the top lip are compensated in such a manner that the width of the headbox slice will remain as constant as possible crosswise, or maintain a certain profile.
A new feature that has been considered in this Finnish patent application, is that the top lip beam is supported onto the frame beam or similar with a set of mechanical actuators, comprising several successive, crosswise actuators. Position transmitters have been arranged at these actuators, which measure the change of position perceived at each actuator. These position transmitters are connected to an electrical control circuit, with which the actuator set is controlled by means of regulating equipment, such as an hydraulic valve.
In the prior art, the headbox slice from which the stock spray is discharged onto the forming wire or into the forming gap, is fine-adjusted by means of a tip strip, to which several parallel adjusting shafts have been connected. With these adjusting shafts, the tip strip is so bent that the thickness profile of the lip spray is suitable, generally as even as possible. In order to make the adjusting of the lip spray possible, the tip strip must be movably supported onto its mating surface, usually onto the front wall of the top lip beam of the headbox.